1. Field of the Invention
The present invention relates to a manufacturing method of an active circuit elements integrated type liquid crystal display having an individual switching transistor for each picture element on the same substrate and an active circuit element operating said switching transistor.
2. Prior Art
AV sets and picture processing software with high image quality have been quickly propagated in each home, and together with development of high vision TV sets, the display is in a trend to become wider and finer.
However, CRT direct-view type or CRT projection type TV sets according to the prior art have a defect that they are very large and heavy. Therefore, it is difficult to further expand the size and increase the image quality.
On the other hand, since the projection type liquid crystal display is possible to significantly reduce its weight and size, it has a very bright future as a next generation display unit and its market is considered to steadily increase.
The projection type liquid crystal display projects the image on a screen by radiating light from the light source through a panel controlling the optical transmission. By activating liquid crystal in each picture element provided in said panel using thin-film transistors, the optical transmission of said panel is controlled. In this projection type liquid crystal display, an approach toward high density picture element has been taken to increase image quality, and in the switching transistors to be provided to each picture element, an approach toward low leakage has been taken to maintain current viased to the liquid crystal.
Simplifying the manufacturing process is also important subject for the panel used for this projection type display because of its complicated manufacturing process. To simplify this manufacturing process and reduce manufacturing cost, an approach widely considered is that the circuit elements activating the switching transistors on the picture element part is formed in one-body on the peripheral area of the picture elements on the same chip by the same process or by corresponding process thereof and the mounting process together with external input-output device and a light valve is simplified.
In the liquid crystal light valve according to the prior art, amorphous silicon thin-film transistors have been used as switching transistors in order to reduce a leakage current. However, if the activating circuit elements are integrated on the same substrate, the On-state current must be high for the transistors constituting the active circuit elements and higher On-state current is required for the picture element transistors.
Therefore, an alternative method using polysilicon thin-film transistors having polysilicon channel in which the mobility is higher about two digits than that of the amorphous silicon has been proposed.
Furthermore, in order to further improve the characteristics of the polysilicon thin-film transistors, improvement of the mobility and threshold voltage by the reduction of the trap-state density using methods such as the laser annealing and the plasma hydrogenation and low leakage and high pressure-proof using the off-set gate structure are aimed.
The prior art will be described hereinafter, referring to FIGS. 1A to 1D. FIGS. 1A to 1D are sectional views showing the manufacturing method of transistors constituting the active circuit elements integrated liquid crystal display according to the prior art. FIG. 1A shows the first process and FIG. 1B the second process. FIG. 1C shows the active circuit transistors of the third process and FIG. 1D the picture element switching transistors of the same.
First, as shown in FIG. 1A, amorphous silicon is deposited on a substrate 1 comprising transparent quartz substrate under temperature at about 500.degree. C. and crystallized by heat treatment at 600.degree. C. under nitrogen atmosphere for about 15 hours to form large crystalline polysilicon layer 2 for an active layer.
By this process, grain boundaries in the active layer region can be reduced and the characteristics can be improved. There is another method available to improved the crystallization of the polysilicon layer 2 and subsequently improve TFT characteristics by heat treatment of the polysilicon layer 2 at least 1000.degree. C.
Then, an active layer part comprising the polysilicon layer 2 is formed by the patterning of the polysilicon layer 2, and a gate oxide film 3 is deposited on the whole surface at the thickness of 100 nm using the CVD technique.
As the next step, another polysilicon layer is deposited on the whole surface at the thickness of 200 nm in order to form a gate electrode 4. After phosphorous is diffused on this polysilicon layer, the gate electrode 4 is formed by the patterning.
Subsequently, as shown in FIG. 1B, the oxide film 3 is etched back so as to remain the gate oxide film 3 immediately under the gate electrode 4 and to remove all other part of the gate oxide film 3. Then, a through oxide film 5 is deposited at the thickness of about 30 nm using the CVD technique.
Then, photoresist having openings for formation of a source region and drain region is provided, and using this photoresist as mask, the source region 6 and the drain region 7 are formed by implantation of boron for P-type transistors or phosphorous for N-type transistors.
The withstanding voltage between the source and the drain can be improved and the leakage current can be reduced by providing an off-set region 8 without ion implantation on the drain side in the transistors constituting the picture element switching transistors and the peripheral active circuit elements at this process so as to form the off-set structure. Further improvement of the active circuit elements also can be achieved by forming the LDD structure implanting phosphorous for the N-type or boron for the P-type in the off-set region 8 at the dose of 5.times.10.sup.12 cm.sup.-2, for example.
Then, as shown in FIGS. 1C and 1D, a layer separating film 9 is deposited at the thickness of about 400 nm, for example, and by heat treatment at about 900.degree. C., impurities are activated and the layer separating film 9 is reflowed.
Subsequently, apertures for contacts 9a are formed on the source region 6 and the drain region 7 of the picture elements and the active circuit transistors by the plasma etching, aluminum is sputtered on the whole surface, then an aluminum electrode 10 is formed by the patterning.
After formation of the aluminum electrode 10, the trapstate density of the channel region is reduced by the plasma hydrogenation (IEEE ELECTRON DEVICE LETTERS, Vol. 10, No. 3, March 1989).
Finally, the layer separating film is deposited and flattened, the second aluminum electrode for the data bus line, and a liquid crystal light valve is formed following the liquid crystal enclosure process.
According to the prior art, the manufacturing method based on using transparent quart for the substrate 1. However, in the process using glass for the transparent substrate 1, low-temperature process is unavoidable since the heat tolerance of the substrate is 600.degree. C. or less. Therefore, improvement of crystallization by high temperature annealing can not be used during its manufacturing process. For this reason, high quality polysilicon film is formed by crystallization of amorphous silicon by the laser annealing, aiming improvement of the transistor characteristics.
Next, defects of the prior art shown in FIGS. 1A to 1D will be explained referring FIGS. 2A to 2C. FIGS. 2A to 2C are drawings showing the characteristics of transistors constituting the active circuit elements integrated liquid crystal display according to the prior art before and after the plasma hydrogenation; FIG. 2A shows the drain current-drain voltage characteristics, FIG. 2B shows the drain current-gate voltage characteristics of the N-type transistors and FIG. 2C shows the drain current-gate voltage characteristics of the P-type transistors.
As aforementioned, there are many trap-state density in polysilicon due to crystallization failure and grain boundaries which contribute to lower characteristics.
Heretofore, in order to reduce the trap-state density in polysilicon film and improve the characteristics, plasma hydrogenation has been used.
In transistors of the active circuit elements, as clearly shown by the drain current-drain voltage characteristics in FIG. 2A, the ON-current is improved from 400 .mu.A to 650 .mu.A in the N-type transistors by plasma hydrogenation while the withstanding voltage between the source and the drain lowered from 33 V to 22 V according to improvement of the mobility and reduction of the threshold voltage. The P-type transistors show similar trend.
This can be considered due to the fact that the holes of the electron-hole pair generated at the drain edge of the N-type transistors accumulate at the source edge so that the parasitic bipolar operation which forward viases the P-N junction at the source side can readily occur.
Once the trap-state density is reduced by plasma hydrogenation, generated holes easily accumulate so that reduction of withstanding voltage by the parasitic bipolar operation becomes remarkable. If the gate length is shortened, the ON-current is improved while the withstanding voltage between the source and the drain is reduced.
As conclusion, higher voltage is required for the drain voltage of transistors constituting the active circuit elements compared to the picture element transistors. However, in order to reduce the area of the light valve, the area occupied by the peripheral active circuit elements must be reduced; that is the gate length must be expanded, on the other hand. Therefore, reduction of withstanding voltage becomes significant problem, particularly in the active circuit elements.
As obvious from FIGS. 2B and 2C, the threshold voltage decreases and the subthreshold swing increases, due to reduction of trap-state density of polysilicon in the active layer by plasma hydrogenation.
FIGS. 1A to 1D explain about transistors having the structure of which the P-type transistor is to be the depletion type MOS transistor, however there are some cases the N-type transistor becomes the depletion type transistor because the threshold voltage of the transistor is influenced by the quality of polysilicon film of the channel region, the film quality of the gate oxide film, process conditions and the like. In case of this N-type transistor, if the trap-state density is reduced by plasma hydrogenation and the like, control of the threshold voltage becomes difficult.
In transistors of the picture element part, however, the gate voltage is about -2 V or less at the OFF-state and therefore less influenced by the depletion as mentioned above in the view point of circuit design.
As mentioned above, in transistors constituting the active circuit with relatively high activation voltage, the withstanding voltage between the source and the drain decreases if the TFT characteristics of the substrate formed in one-body with the active circuit elements is improved by plasma hydrogenation and the like to reduce the leakage current of the picture element transistors. Therefore, it is difficult to apply this process to transistors having shorter gate length, and therefore regarding the ON-current the effects of the mobility increase and the decrease of threshold voltage by reduction of the trap-state density offset each other. Furthermore, the size of transistor becomes larger and the area occupied by the transistors becomes wider, which contravene the requirement for reduction of the size of light valve.
Further, there are several cases either the P-type transistor or the N-type transistor comes close to the depletion type and the OFF-current becomes higher by decrease of the threshold voltage and increase of the subthreshold swing, therefore these are significant problems for transistors particularly constituting the active circuit elements.